We sought to identify an evident enthalpic-driven transition below an entropic-driven closed-loop phase transition by studying the phase behavior in miscible binary block copolymer (BCP) blends of a poly-styrene-b-poly (n-butyl methacrylate) (PS-b-PnBMA) and a deuterated polystyrene-b-poly (n-hexyl methacrylate) (dPS-b-PnHMA), using small-angle x-ray scattering (SAXS) and depolarized light scattering (DPLS). Intriguingly, an order-to-disorder transition (ODT) below the closed-loop transition consisting of a lower disorder-to-order transition (LDOT) and an upper order-to-disorder transition (UODT) was observed in the BCP blends of an asymmetric (PS-rich) PS-b-PnBMA and a symmetric dPS-b-PnHMA. Multiple phase transitions, previously undiscovered in BCP system, were attributed to a delicate balance in the free energy of the dispersive intermolecular interactions, the entropic compressibility, and the combinatorial entropy of mixing at an experimental temperature range between the glass transition temperature (T-g) and degradation temperature (T-d). We also characterized the effects of hydrostatic pressure on the transition temperatures to determine the thermodynamic origins of each phase transition. (C) 2017 Elsevier Ltd. All rights reserved.